1. Field of the Invention
The present invention relates to a digital audio amplifier, and more particularly, to a demodulation apparatus for a digital audio amplifier.
2. Description of the Related Art
Generally, the purpose of a digital audio amplifier is to minimize distortion of a signal and to output high fidelity audio signals by digitally processing a received audio signal.
Referring to FIG. 1, which indicates a block diagram of a conventional digital audio amplifier, a digital audio amplifier 104 receives a digital audio signal and amplifies it in a digital state.
As shown in FIG. 2, the digital audio amplifier 104 includes a micro controller 202, a digital audio signal conversion section 204, and an amplification output section 214. The micro controller 202 controls a sound controller 208 of the digital audio signal conversion section 204 so as to provide effective acoustic control.
An input section 206 of the digital audio signal conversion section 204 selects any one of a plurality of input signals or transforms the sampling rate of the selected digital audio signal. The sound controller 208 is composed of a digital signal processor (DSP), and performs tone control, graphic equalizing, volume control, sound field control or the like, under the control of the micro controller 202. An oversampling digital filter 210 artificially increases the sampling frequency by use of interpolation in order to facilitate the design of a low-pass filter that is a demodulation apparatus 106 and to improve a ratio of S/N (signal to noise). A PCM-PWM conversion section 212 is a DDC (digital to digital converter) so that a PCM-type digital audio signal is converted into a PWM-type digital audio signal.
The amplification output section 214 is composed of a gate driver 216 and a MOSFET bridge circuit 218. The gate driver 216 outputs a signal to drive the MOSFET bridge circuit 218, and has a high output current and a low output impedance.
The MOSFET bridge circuit 218 outputs a high output voltage and a high current so as to apply the high current to a load having a low impedance, such as a speaker. Since the PWM-type digital audio signal is a square wave having a high frequency, an MOSFET is used as an output device.
The demodulation apparatus 106 shields a high frequency component and passes a low frequency component only, both of which are contained in the PWM-type digital audio signal, so as to demodulate the PWM-type digital audio signal into an analog audio signal.
The demodulation apparatus 106 that is employed in the digital audio amplifier and that uses an analog-to-digital (AD) drive type digital audio signal will now be described with reference to FIG. 3.
The demodulation apparatus 106 is composed of two low-pass filters, first low-pass filter A and second low-pass filter B. The first low-pass filter A, which is interposed and connected between a positive digital audio signal output terminal and one end of the load, is composed of a first inductor L1 and a first capacitor C1. The first low-pass filter A demodulates the positive digital audio signal into an analog audio signal and outputs the result to one end of the load. The second low-pass filter B, which is interposed and connected between a negative digital audio signal output terminal and the other end of the load, is composed of a second inductor L2 and a second capacitor C2. The second low-pass filter B demodulates the negative digital audio signal into the analog audio signal and outputs the result to the other end of the load. In the AD drive type, the negative digital audio signal is the reversed shape of the positive digital audio signal.
Since the demodulation apparatus 106 employed in the digital audio amplifier includes a plurality of inductors L1 and L2, problems exist in terms of limited space, high costs, and heat generation in the digital audio amplifier. In order to cope with the heat generation in the digital audio amplifier, the conventional amplifier employs components with high heat resistance or a plurality of heat sinks. Accordingly, the dimensions or costs of the digital audio amplifier may not be reduced all the more.